BOSTON – Courtney Lee did not lash out about his frustrations last season, but privately the Boston Celtics guard stewed. About what, he did not want to discuss Friday. But he was so upset he allowed his work ethic to slip.

“I was wondering a lot (about his shot). I really wasn’t doing too much staying after and working on it,” Lee said Friday night at the TD Garden before Boston’s 106-98 victory against Denver. “I was just trying to get by off talent and God-given ability. This summer I got back to what I was doing previous years, as far as not taking a break, working out every day, playing as much as possible. I got a new trainer, staying after practice and continuing to work.”

The guard would not specify exactly what upset him last year, saying, “Just a lot of reasons. I was frustrated with my performance, with a lot of other things.”

When one reporter prodded further, Lee replied, “There was a lot of things in particular, but I don’t want to talk about them.”

Lee began last season as a starter but didn’t stay in the first unit for long. His role sometimes fluctuated drastically from one night to the next, and he was basically out of Doc Rivers’ rotation entirely by the playoffs. Lee appeared in just four of the six postseason games, and received fewer than 10 combined minutes in two others.

“I started a lot of games, I came off the bench, and then towards the end of the season I wasn’t playing,” he said. “So it was definitely difficult. The more consistent you get with rotations and knowing what’s going on, and get familiar with the guys that you’re going to be playing with on a night-in-and-night-out basis, the more comfortable and consistent you’ll be.”

Interestingly enough, Lee is playing about seven fewer minutes per game this season. But he sounds more comfortable knowing where he fits, which he didn’t always under Rivers.

“As you get older, the less minutes and the more consistent minutes that you get, it’s kind of helping,” Lee said. “Everybody that’s going out there playing is doing a good job. (Brad Stevens) is a new coach, so I’m sure it’s tough on him. I just want to continue to go out there and make the impact in the time that I’m out there.”

Ironically, Lee was asked plenty about his improved jump shot and confidence ... and followed it with his second scoreless game of the season. The first came two weeks ago against the Memphis Grizzlies, when he tried to play through a sore left knee that kept him out of the following two contests.

Even after his 0-for-3 night Friday, Lee, at 50.9 percent from the field and 44.8 percent from deep, is having the best shooting season of his career. But he remembers the hitch he developed last year, which he worked hard with trainer Rob McClanaghan to remove.

“Someone said I had the same shot as Michael Kidd-Gilchrist,” Lee said. “That was the biggest – not a knock on him, but he has a little hitch in his shot. So I was like, ‘Come on, man.’ I had to get that out of my game.”